Elsevier

The Lancet

Volume 359, Issue 9321, 1 June 2002, Pages 1896-1903
The Lancet

Articles
Sexual mixing patterns and sex-differentials in teenage exposure to HIV infection in rural Zimbabwe

https://doi.org/10.1016/S0140-6736(02)08780-9Get rights and content

Summary

Background

HIV-1 prevalence typically rises more rapidly at young ages in women than in men in sub-Saharan Africa. Greater susceptibility to infection on exposure in women is believed to be a contributory factor as is greater exposure to previously infected sexual partners of the opposite sex. We investigated the latter hypothesis using data from a field study in rural Manicaland, Zimbabwe.

Methods

Quantitative data on onset and degree of sexual activity, numbers of partners, concurrent partnerships, condom use, and partner characteristics were used in conjunction with epidemiological data on age and sex specific prevalence of HIV infection to do statistical analyses of association between key variables. Mathematical models and qualitative data were used to aid analysis and interpretation.

Findings

Older age of sexual partner was associated with increased risk of HIV-1 infection in men (odds ratio 1·13 [95% CI 1·02–1·25]) and women (1·04 [1·01–1·07]). Young women form partnerships with men 5–10 years older than themselves, whereas young men have relationships with women of a similar age or slightly younger. Greater number of lifetime partners is also associated with increased risk of HIV (1·03 [1·00–1·05]). Young men report more partners than do women but infrequent coital acts and greater use of condoms. These behaviour patterns are underpinned by cultural factors including the expectation that women should marry earlier than men. A strong gender effect remains after factors that affect exposure to infected partners are controlled for (6·04 [1·49–24·47]).

Interpretation

The substantial age difference between female and male sexual partners in Manicaland is the major behavioural determinant of the more rapid rise in HIV prevalence in young women than in men. Theoretical studies have suggested that this difference is an important determinant of observed epidemiological patterns but the study reported in this paper provides clear empirical evidence of association.

Introduction

Data from antenatal surveillance1 and population2, 3, 4, 5 surveys in sub-Saharan Africa reveal extremely high levels of HIV infection in teenage women. By contrast, infection rates in men remain low until their mid-to-late 20s. Development of strategies that prevent the early build-up of HIV infection in young women is a key public-health priority. However, this requires a sound understanding of the biomedical, behavioural, cultural, and socioeconomic processes that contribute to observed patterns of infection.

As with other sexually transmitted diseases (STDs),6 the probability of male-to-female sexual HIV transmission is greater than that from female to male.7 However, mathematical-model simulations indicate that differences in exposure to infection, due to contrasting patterns of sexual behaviour and location within sexual networks, also affect patterns of spread of STDs.8, 9, 10, 11, 12 Figure 1 shows how more extensive sexual mixing between young women and older men can increase the sex ratio of HIV prevalence at ages 15–19 years for a plausible range of ratios of male-female:female-male HIV transmission probabilities in a generalised HIV epidemic simulated in a heterosexual, sexual activity-stratified population. The model13 simulations were generated by varying the ratio of male-female versus female-male transmission while holding the geometric mean of the two constant. From a baseline scenario in which the pattern of sexual partner choice according to age is assortative (like-with-like), the proportion of women who would otherwise have had a partner of the same age but preferentially chose one 5–10 years older was varied systematically along with the reciprocal for men, in the reverse direction. Sexual activity was assumed to commence at age 15 years for both sexes.

Similar sex-patterns and age-patterns of HIV infection have been noted in rural Zimbabwe14 where the HIV epidemic probably began to spread in the late 1980s15 and HIV prevalence remains very high.14 One in four adults is currently infected, a level which, if maintained, means that a young person entering the sexually-active population today will have a two out of three chance of acquiring HIV infection before reaching his or her 55th birthday.12 Clearly the protection of future generations from this tragedy is a crucial goal.

To identify current risk behaviours for adult HIV infection in rural Zimbabwe, we did a random household survey of 9843 adults in the eastern province of Manicaland. Here we use quantitative data on sexual behaviour from 4429 young men and women and findings from parallel qualitative research to investigate the behavioural risk factors for HIV infection in young people and to identify factors contributing to the higher incidence in young women.

Section snippets

Participants and procedure

Individual-based stochastic model simulations of networks of sexual partnerships and STD transmission suggest that, in advanced stages of an epidemic, measures of risk behaviour accumulated over periods similar to the duration of infection will be the best predictors of individuals' infection.16 For HIV, for which the incubation period for AIDS is long, measures such as age at first sexual intercourse and number of lifetime partners will be better indicators of exposure to infection—and, thus,

Results

98% (8233 of 8386) of households were included and 4419 men (2153 younger than 25 years of age, 75% of those eligible) and 5424 women (2276 [73%]) joined the study. 8% of respondents were nonliterate.

Women aged 15–24 years are considerably more likely to be infected than men of the same age (age-adjusted odds ratio 4·62 [95% CI 3·65–5·84]; p<0·001; figure 2). In women, HIV infection commences at around age 16 years and incidence is extremely high throughout the late teens and early 20s. In men,

Discussion

It has long been suspected that younger women having relationships with older men contributes to the spread of HIV infection in young women.17, 23 However, we describe and provide empirical evidence that shows this effect. This aspect of sexual partner networks has a pivotal role in the persistence of major HIV epidemics because not only do large segments of successive cohorts of young women become infected through this route, but many further infections result when these women marry and have

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